The present invention generally relates to a semiconductor device and, more particularly, to a semiconductor bipolar junction transistor (BJT) device with an enhanced beta gain.
In a semiconductor BJT device, by controlling the voltages applied to its base and collector terminals, the device may operate in a forward-active mode. Taking an NPN-type BJT device as an example, i.e., a BJT device with a P-type base region and N-type collector and emitter regions, in operation, a positive voltage VBE and a positive voltage VCE higher than VBE may be applied to the base terminals and the collector terminals, respectively. The emitter-base junction may be therefore forward-biased and the base-collector junction may be therefore reverse-biased, and a base current IB and a collector current IC which by definition is βF times the base current IB may be induced. The BJT device may therefore serve as a current amplifier with a current gain or beta gain βF.
FIG. 1A is a schematic cross-sectional view of a BJT device 1-1 in prior art. Referring to FIG. 1A, the BJT device 1-1 may include base terminals B, emitter terminals E and collector terminals C. When these terminals are appropriately biased, the junction between the base region and the emitter region, i.e., the emitter-base junction, may be forward-biased and the junction between the base region and the collector region, i.e., the base-collector junction, may be reverse-biased. The BJT device 1-1 may therefore operate in a forward-active mode, in which base currents IB may be induced and flow in a base region from the base terminals B via an emitter region to the emitter terminals E. Meanwhile, collector currents IC may be also induced and flow in a collector region from the collector terminals C via the base and emitter regions to the emitter terminals E, wherein the collector current IC is βF times the base current IB. The beta gain βF may be a function of the area of the base region. For example, the current gain βF may be increased as the base area of the BJT device 11-1 is decreased. However, the distance W1 between a P+ region associated with the base terminals and an N+ region associated with the emitter terminals and the distance W2 between the P+ region and the collector-base junction may be subject to a design rule limit.
FIG. 1B is a layout of the BJT device 1-1 illustrated in FIG. 1A. Referring to FIG. 1B, the emitter region, i.e., the N+ region under the emitter terminals E, may be formed in a square pattern and the P+ region under the base terminals B may be formed in a ring pattern. The P+ region may substantially surround the emitter region and may be separated from the emitter region by W1. Furthermore, the P+ region may be separated from the collector-base junction by W2. Due to the limits of W1 and W2, it may be difficult to directly downsize the area of the base region of the BJT device 1-1 in order to increase the beta gain without violating the design rules.
It may therefore be desirable to have a BJT device with a new layout that may achieve a downsized base area and in turn an enhanced beta gain while observing the design rules.